My final project is the effect of hydrothermal holding time to zeolite characterization which synthesized by geothermal sludge. Zeolite is a smart material. It is belong to microporous material and has many advantages such as filter, adsorbent, ion exchanger and the others. I took the gothermal sludge from geodipa energy, the owner of geothermal plant in Dieng, Wonosobo. Geothermal sludge was dried and was distilled, and then fused at 850 deg C. Geothermal ash mixtured with natrium aluminate and natrium silicate to get an aluminosilicate gel. It was synthesized hydrothermally at 100 deg C during 3,4,and 5 hours.Then,it was washed by aquabidest and was dried in the oven at 80 deg C during 5 hours. Finelly zeolite was produced by the process.